This invention relates to a power transmission mechanism mounted on a vehicle for transmitting driving torque to a wheel of the vehicle, and an electric drive system including two such power transmission mechanisms for driving at least the front wheels or the rear wheels of a vehicle.
Because of their environment-friendliness, electric vehicles, which run on an electric motor alone, and hybrid cars, which use both a conventional internal combustion engine and an electric motor, are now being vigorously developed by many car manufacturers. Some of them are already hitting the market. The electric drive system of this type of vehicles has a power transmission mechanism in its drive train. Such a power transmission mechanism includes a clutch for selectively allowing and shutting off torque transmission between the driving motor and wheels to protect the motor against overloads from the wheels or prevent accidents due to runaway of the motor. Conventional such clutches include a dog clutch, which includes claws provided on the driving member and driven members and adapted to be meshed with each other, and a friction clutch, which includes friction surfaces provided on the driving and driven members and adapted to be brought into frictional contact with each other (see JP patent publications 11-91389 and 2001-287550 (both unexamined)).
When engaging a dog clutch, it is necessary to extremely accurately synchronize the rotations of the driving member and the driven member. Otherwise, the claws of the driving and driven members may fail to mesh with each other and/or shock may be produced when they are meshed with each other. Even if the claws are meshed with each other, they may be locked together due to torque resulting from friction in the motor. Once they are locked together, they may not easily separate from each other even when trials are made to disengage the clutch after the motor has been stopped.
A friction clutch is free of this problem. But it has another problem in that even when the clutch is disengaged, the driving member and the driven member cannot be completely separated from each other due to the presence of viscous fluid in the clutch. Thus, even when the clutch is disengaged, torque may be transmitted from the wheels to the motor, thus overloading the motor. Thus, such a clutch cannot sufficiently protect the motor.
Most of such conventional clutches are provided in the drive train between the motor and the power distribution mechanism. In a hybrid car, the electric motor and the engine are alternately or simultaneously used to drive the vehicle. When the vehicle is driven only by the engine, the abovementioned clutch is disengaged to separate the electric motor from the wheels. But even in this state, the elements of the drive train between the clutch and the wheels are rigidly coupled to the wheels. Thus, the driving torque from the engine has to be used to drive not only the wheels but also these elements (torque needed to drive these elements is hereinafter referred to as drag torque). This worsens fuel economy. If the clutch is a friction clutch, since such a clutch cannot be completely disengaged as described above, the engine torque will have to be used to drive the motor, too. Also, in order to transmit a sufficiently large driving torque to the wheels from the motor, the friction clutch has to have a large number of friction disks. Such a friction clutch is inevitably large in size and heavy in weight, and thus further worsens fuel economy.
The applicant of this invention has proposed an improved clutch assembly to be used as the power transmission mechanism of the above-described type. This clutch assembly comprises a two-way clutch and an electromagnetic clutch. The two-way clutch includes a driving member having cam surfaces on the outer periphery thereof, a driven member fitted around the driving member and having a cylindrical surface on the inner periphery thereof, a plurality of rollers disposed between the cam surfaces and the cylindrical surfaces, and a retainer disposed between the driving member and the driven member for retaining the rollers. The rollers are capable of engaging both of the driving member and the driven member when the retainer rotates in either rotational direction relative to the driving member. The electromagnetic clutch includes an electromagnet which can be selectively energized and deenergized, and a rotor coupled to the driven member and adapted to be nonrotatably coupled to the retainer when the electromagnet is energized or deenergized (see JP patent publications 2000-326749 and 2003-32806 (both unexamined)).
If such a clutch assembly is provided in the drive train of a vehicle, its two-way clutch can be far more easily engaged and disengaged than a dog clutch, and also, unlike a friction clutch, the motor can be completely separated from wheels. Thus, this clutch assembly can reliably protect the motor. In applying it to a hybrid car, two such clutch assemblies are preferably mounted in the drive train, each near the hub bearing of one of front or rear wheels to minimize drag torque and thus improve fuel economy. If such clutch assemblies protrude from the outboard ends of the respective hub bearings, they will pose safety problems and ruin the appearance of the vehicle. If they protrude from the inboard ends of the hub bearings, they will interfere with driving members such as constant-velocity joints, thus restricting the freedom of design of these members. Thus, at least the two-way clutch of each clutch assembly is preferably mounted radially inside of the hub bearing.
But in order to mount the two-way clutch radially inside of the hub bearing, it is necessary to use small-diameter rollers. The contact area between such small-diameter rollers and the inner and outer members (driving and driven members) of the hub bearing is small. Thus, the maximum torque that can be transmitted through such small-diameter rollers is also small.
In order to increase the contact area and thus the maximum torque that can be transmitted through such small-diameter rollers, it is necessary to increase the number of rollers in each row, or increase the length of each roller. But if the number of rollers in each row is increased, the circumferential area where each roller is disengaged correspondingly shortens. This increases the possibility of malfunction of the clutch. Specifically, this increases the possibility of the rollers inadvertently engaging or disengaging when the retainer rotates even slightly relative to the driving member due to vibrations or shocks. Also, if the number of rollers in each row is increased, it is necessary to correspondingly increase the number of cam surfaces. This increases the cost for forming the cam surfaces and also makes it more difficult to form the cam surfaces. If longer rollers are used to increase the contact area, the rollers are more likely to inadvertently engage or disengage when the inner and outer members of the clutch deflect due e.g. to impulse loads that act on the wheel and as a result, the shape of the gap between the inner and outer members changes.
An object of the invention is to provide a power transmission mechanism including a clutch which can be easily engaged and disengaged, which can completely shut off transmission of torque between the driving member and the driven member, and which can tranmit sufficiently large torque.